In the related art, it is desired for a transmitter/receiver provided in a mobile communication base station for microwave-band communication to reduce power consumed by the transmitter/receiver and reduce the size and the cost of the transmitter/receiver. In order to achieve such goals, it is effective to reduce a pass loss of power caused in a unit of the transmitter/receiver after an amplifier circuit, that is, between the amplifier circuit and a transmission/reception antenna.
For the unit of the transmitter/receiver after the amplifier circuit, an SWR (Standing Wave Ratio) is measured using a transmission wave (traveling wave) output from the amplifier circuit during a transmission period and a reflected wave which is the traveling wave reflected in a cable by the transmission/reception antenna, or the like. The SWR represents the ratio of any reflected wave, which is reflected because of a mismatch between the impedance of the transmission/reception antenna connected to the cable and the impedance of the cable to the traveling wave.
A standing wave ratio measuring circuit according to the related art that measures the SWR described above will be described with reference to FIGS. 15 and 16. FIGS. 15 and 16 illustrate the standing wave ratio measuring circuit according to the related art. FIG. 15 is a block diagram illustrating the configuration of the standing wave ratio measuring circuit. FIG. 16 is a timing chart illustrating operation timing.
A standing wave ratio measuring circuit 9 includes a PA (Power Amplifier) 91, a CIR (Circulator) 92, and a detection circuit 93 that detects a traveling wave and a reflected wave. The standing wave ratio measuring circuit 9 further includes a BPF (Band Pass Filter) 94, a transmission/reception antenna 95, an SWR computing circuit 96, an ISO (Isolator) 97, and an LNA (Low Noise Amplifier) 98.
The PA 91 amplifies a transmission signal in a radio frequency band generated by a transmission processing unit (not illustrated). The CIR 92 supplies the high-power transmission wave amplified by the PA 91 to the detection circuit 93 during a transmission period, and supplies a reception wave supplied from the detection circuit 93 to the ISO 97. The detection circuit 93 includes two DCs (Directional Couplers) (DC1 and DC2) for high power usage.
The DC1 distributes the high-power transmission wave supplied from the CIR 92 to the SWR computing circuit 96 as a traveling wave, and also distributes the transmission wave to the DC2. The DC2 supplies the transmission wave distributed from the DC1 to the transmission/reception antenna 95 via the BPF 94. In a case where the supplied transmission wave is reflected at a location after the BPF 94, the DC2 detects the reflected wave via the BPF 94 to distribute the reflected wave to the SWR computing circuit 96, and also to supply the reflected wave to the DC1 as a reception wave.
The SWR computing circuit 96 measures the SWR using the traveling wave distributed from the DC1 and the reflected wave distributed from the DC2. The ISO 97 supplies the reception wave supplied from the CIR 92 to the LNA 98. The LNA 98 amplifies the reception wave supplied from the ISO 97 to supply the amplified reception wave to a reception processing unit (not illustrated) as a reception signal.
In the standing wave ratio measuring circuit 9, as illustrated in FIG. 15, a reception period and a transmission period are assigned in a time-sharing manner. During a transmission period, that is, when the operation mode is a transmission mode, the transmission wave is propagated, and at the same time, the SWR is measured using the traveling wave and the reflected wave. Meanwhile, during a reception period, that is, when the operation mode is a reception mode, the reception wave is propagated. Related techniques are disclosed in documents including Japanese Laid-open Patent Publication No. 2008-147934 and Japanese Laid-open Patent Publication No. 2004-286632.
In the standing wave ratio measuring circuit 9 according to the related art, part of the traveling wave and the reflected wave propagated during the transmission period and the reception period assigned in a time-sharing manner is extracted by the directional couplers (DC1 and DC2) at a location after the PA 91, which may increase the pass loss of power.
That is, the standing wave ratio measuring circuit 9 distributes a part of the transmission wave amplified by the PA 91 to the SWR computing circuit 96 using the DC1 as a traveling wave during the transmission period, and thus a radio wave corresponding to an amount for distribution is removed from the transmission wave, which increases the loss in power for that amount of the transmission wave. Also, the standing wave ratio measuring circuit 9 distributes a part of the reception wave supplied from the transmission/reception antenna 95 to the SWR computing circuit 96 using the DC2 during the reception period, and thus a radio wave corresponding to an amount for distribution is removed from the reception wave, which increases the loss in power for that amount of the reception wave.
Moreover, the transmission wave output from the PA 91 has high power, and thus the power gain of the transmission wave may be varied in some cases. In such cases, the transmission wave is distorted, as a result of which the signal level of the transmission wave (traveling wave) and the reflected wave for the traveling wave may not be detected accurately, which could increase errors in the measurement of the SWR.
The above events occur not only in mobile communication base stations, but also in radio apparatuses in general which include a transmission/reception antenna and in which the standing wave ratio is measured.